U.S. patent number 7,038,853 [Application Number 10/745,830] was granted by the patent office on 2006-05-02 for athermalized plastic lens.
This patent grant is currently assigned to Symbol Technlogies, Inc.. Invention is credited to Edward Barkan, Vladimir Gurevich, Mark Krichever, Yajun Li, Miklos Stern.
United States Patent |
7,038,853 |
Li , et al. |
May 2, 2006 |
Athermalized plastic lens
Abstract
A plastic lens includes refractive and diffractive optical
apparatus configured to produce optothermal changes substantially
canceling each other over a predetermined working temperature range
to render the plastic lens substantially athermalized over the
range.
Inventors: |
Li; Yajun (Oakdale, NY),
Gurevich; Vladimir (Stony Brook, NY), Krichever; Mark
(Hauppague, NY), Barkan; Edward (Miller Place, NY),
Stern; Miklos (Woodmere, NY) |
Assignee: |
Symbol Technlogies, Inc.
(Holtsville, NY)
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Family
ID: |
27537225 |
Appl.
No.: |
10/745,830 |
Filed: |
December 24, 2003 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20040136069 A1 |
Jul 15, 2004 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09551272 |
Apr 18, 2000 |
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09109018 |
Jul 1, 1998 |
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08953543 |
Oct 20, 1997 |
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08624935 |
Mar 22, 1996 |
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08173255 |
Dec 27, 1993 |
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07860390 |
Mar 30, 1992 |
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Current U.S.
Class: |
359/566;
235/462.22; 235/462.35; 359/569 |
Current CPC
Class: |
G02B
3/08 (20130101); G06K 7/10811 (20130101); G02B
5/001 (20130101) |
Current International
Class: |
G02B
27/44 (20060101) |
Field of
Search: |
;359/566,569,570,721,741,742,743 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
C Londono, W. T. Plummer, P. P. Clark, `Athermalizationof a
single-component lens with diffractive optics`, Appl. Opt., vol.
32, No. 13, May 1, 1993, pp. 2295-2302. cited by examiner .
Carts, "Micro-optics has macro potential," Laser Focus World, Jun.
1991. cited by other .
Figiwara, "Optical properties of conic surfaces, " I Reflecting
Cone, J. Opt. Soc. Am., 52, 287-292 (1962). cited by other .
Goodman, Introduction to Fourier Optics, Table of Contents, pp.
110-120, first ed. 1968, second ed. 1996. cited by other .
Johnson et al., "Connectorized Optical Link Package Incorporating a
Microlens," Proceedings of the 30th Electronics Components
Conference, San Francisco, CA, Apr. 28-30, 1980. cited by other
.
Tsi et al., "System analysis of CCD-based bar code readers," Appl.
Opt., 32, 3504-3512 (1993). cited by other .
Behrmann et al., "Influence of temperature on diffractive lens
performance," Appl. Opt., 32, 2483-2489. cited by other.
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Primary Examiner: Nguyen; Thong
Assistant Examiner: Lavarias; Armel C.
Attorney, Agent or Firm: Watts Hoffmann Co., L.P.A.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No.
09/551,272, filed Apr. 18, 2000, now abandoned which is a
continuation of U.S. application Ser. No. 09/109,018, filed Jul. 1,
1998, now abandoned which is a continuation-in-part of U.S.
application Ser. No. 08/953,543, filed Oct. 20, 1997, now abandoned
which is a continuation-in-part of U.S. application Ser. No.
08/624,935, filed Mar. 22, 1996, now abandoned which is a
continuation-in-part of U.S. application Ser. No. 08/173,255, filed
Dec. 27, 1993, now abandoned which is a divisional of U.S.
application Ser. No. 07/860,390, filed Mar. 30, 1992 now abandoned.
The six aforementioned applications are incorporated by reference
herein in their entirety.
Claims
What is claimed is:
1. A plastic lens for use in a bar code scanner that acts upon
light reflected from a bar code symbol, comprising: a refractive
and diffractive optical apparatus for use as a lens in the bar code
scanner; the optical apparatus configured to produce optothermal
changes substantially canceling each other over a predetermined
working temperature range to render the plastic lens substantially
athermalized over the range, and wherein the lens includes an
axicon; and wherein the axicon is configured to have an induced
phase coefficient that elongates the focal depth of the lens based
on an expected spatial wavelength of the bar code symbol.
2. The lens of claim 1, comprising a refractive surface and a
diffractive optical element, wherein optothermal changes due to the
refractive surface counter optothermal changes due to the
diffractive optical element.
3. The lens of claim 1, wherein the optothermal changes canceling
each other include changes affecting the focal length of the
lens.
4. The lens of claim 1, comprising polycarbonate.
5. The lens of claim 1, comprising acrylic.
6. The lens of claim 1, wherein the lens has a net positive
power.
7. The lens of claim 1, wherein an optothermal expansion
coefficient of the refractive optical apparatus is higher than an
optothermal expansion coefficient of the diffractive optical
apparatus.
8. The lens of claim 1, comprising a diffractive optical element
that is substantially smaller than the lens.
9. The lens of claim 1, wherein a first surface of the lens
provides substantially all of the negative power of the lens, and a
second surface of the lens provides substantially all of the
positive power of the lens.
10. The lens of claim 1, wherein a surface of the lens provides
substantially all of the negative power of the lens and
substantially all of the positive power of the lens.
11. The lens of claim 1, wherein the diffractive optical apparatus
includes a diffractive optical element that is substantially
spherical in average.
12. The lens of claim 1, wherein a surface of the lens is
substantially flat.
13. The lens of claim 1, wherein the refractive optical apparatus
is divided unevenly between first and second surfaces of the
lens.
14. The lens of claim 1, wherein substantially all of the
diffractive optical apparatus is disposed on one surface of the
lens.
15. The lens of claim 1, wherein the diffractive optical apparatus
is divided substantially evenly between first and second surfaces
of the lens.
16. The lens of claim 1, wherein said bar code scanner comprises a
CCD-imager.
Description
BACKGROUND OF THE INVENTION
The invention relates to an athermalized plastic lens.
In a system (e.g., a bar code scanner) that relies on a specific
optical property (e.g., a specific focal length) of a lens, changes
in temperature that affect the specific optical property of the
lens can cause the system to function improperly or inaccurately.
For example, if the lens is used in a bar code scanner to focus
light reflected from a bar code symbol onto a CCD device that
produces an image of the symbol, the produced image may be too
out-of-focus to be effectively decoded if the focal length of the
lens is affected significantly by a temperature change. Typically,
a glass lens is more resistant to temperature changes than a
plastic lens having the same shape.
SUMMARY OF THE INVENTION
The invention provides an athermalized plastic lens in which
optothermal changes are balanced by refractive and diffractive
optics, allowing the lens to achieve thermal performance
characteristics similar to those of a glass lens, while being
inexpensive, lightweight, and easily shaped. When the lens includes
an axicon, the lens provides equipment such as bar code scanners
with an extended working range.
Preferred implementations of the invention may include one or more
of the following. The lens may include a refractive surface and a
diffractive optical element, wherein optothermal changes due to the
refractive surface counter optothermal changes due to the
diffractive optical element. The optothermal changes may cancel
each other and include changes affecting the focal length of the
lens. The lens may include polycarbonate. The lens may include
acrylic. The lens may include a net positive power. The optothermal
expansion coefficient of the refractive optical apparatus may be
higher than an optothermal expansion coefficient of the diffractive
optical apparatus. The lens may include a diffractive optical
element that is substantially smaller than the lens. The first
surface of the lens may provide substantially all of the negative
power of the lens, and the second surface of the lens may provide
substantially all of the positive power of the lens. The surface of
the lens may provide substantially all of the negative power of the
lens and substantially all of the positive power of the lens. The
diffractive optical apparatus may include a diffractive optical
element that is substantially spherical in average. The surface of
the lens may be substantially flat. The refractive optical
apparatus may be divided unevenly between first and second surfaces
of the lens. Substantially all of the diffractive optical apparatus
may be disposed on one surface of the lens. The diffractive optical
apparatus may be divided substantially evenly between first and
second surfaces of the lens. The lens may include an axicon. The
axicon may include a polymer. The axicon may be disposed at a
substantially spherical surface of the lens. The diffractive
optical element and the axicon may be disposed at different
surfaces of the lens. The lens may include a diffractive optical
element that includes at least eight phase levels. The lens may
include a diffractive optical element that includes fewer than nine
phase levels. The axicon may be affixed to a surface of the lens.
The lens may include an aspherical surface having the optical
properties of a combination of a spherical surface with the axicon.
The lens may include a doublet. The lens may include a Cook triplet
anastigmat. The lens may include a symmetric double Gaussian. The
MTF of the lens may be higher with the axicon than without the
axicon for bar code symbols having spatial wavelengths of 10 20
mils, inclusive. The MTF of the lens may be at least 0.2 for a 10
mil bar code symbol that is from about 4 to about 16 inches away
from the lens.
Other advantages and features will become apparent from the
following description and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of an embodiment of an athermalized
plastic lens having refractive surfaces and diffractive optical
elements.
FIGS. 2a and 2b are illustrations of diffractive optical elements
that are used in embodiments of the athermalized plastic lens.
FIGS. 3 and 4 are illustrations of embodiments of the athermalized
plastic lens.
FIG. 5A is a conceptual illustration of an embodiment of the
athermalized plastic lens having an axicon.
FIG. 5B is an illustration of the embodiment of FIG. 5A.
FIG. 6 is a flat-profile illustration of a diffractive optical
element used in the embodiment of FIGS. 5A 5B.
FIG. 7 is an illustration of another embodiment of the athermalized
plastic lens having an axicon.
FIG. 8 is an illustration of bar code scanning using an
athermalized plastic lens having an axicon.
FIGS. 9A, 9B, 10A, 10B, 11A, and 11B show MTF curves for
athermalized plastic lenses having different axicons.
FIGS. 12 and 13 show MTF curves for different spatial wavelengths
used with athermalized plastic lenses having different axicons.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a lens 10 that is an embodiment of an
athermalized plastic hybrid lens ("hybrid lens") that includes
refractive and diffractive optics. As described below, by balancing
changes in optical properties resulting from temperature-induced
expansion or contraction of lens material ("optothermal changes"),
the hybrid lens achieves thermal performance characteristics
similar to those of a glass lens, while being inexpensive,
lightweight, and easy to shape. The balancing is accomplished by
special properties of surfaces and elements of the hybrid lens
(e.g., spherical refractive surfaces 12, 14 and diffractive optical
elements ("DOEs") 16, 18 of lens 10), as described below.
In at least some cases, the optothermal changes resulting from a
temperature change produce a focal length difference. For a
particular lens, the nature of the relationship between the
temperature change and the focal length difference depends on the
characteristics of the lens. In an athermalized lens, the
temperature change produces no significant focal length difference,
i.e., the Focal length of an athermalized lens is not significantly
affected by temperature changes.
Lens 10 has a focal length f that includes the following components
that are related as described in equation (1) below: a refractive
focal length f.sub.r due to the refractive surfaces 12, 14 which
have focal lengths f.sub.r1 and f.sub.r2, respectively, and a
diffractive focal length f.sub.d due to the DOEs 16, 18 which have
focal lengths f.sub.d1 and f.sub.d2, respectively.
1/f=(1/f.sub.r1+1/f.sub.d1)+(1/f.sub.r2+1/f.sub.d2)=1/f.sub.r+1/f.sub.d
(1)
The refractive surfaces 12, 14 and DOEs 16, 18 have opto-thermal
expansion coefficients x.sub.r and x.sub.d, respectively, each of
which is a measure of the extent to which the respective focal
length (f.sub.r or f.sub.d) is changed per unit of temperature
change. Equation (2) below relates changes .DELTA.f,
.DELTA.f.sub.r, and .DELTA.f.sub.d in focal lengths f, f.sub.r, and
f.sub.d, respectively, to a temperature change .DELTA.T.
.DELTA..times..times..times..DELTA..times..times..times..DELTA..times..ti-
mes..times..times..times..DELTA..times..times. ##EQU00001##
Since lens 10 is athermalized, focal length change .DELTA.f may be
taken to be zero, to produce equation (3) which shows that in lens
10 the ratio of expansion coefficient x.sub.r to focal length
f.sub.r is balanced by the ratio of expansion coefficient x.sub.d
to focal length f.sub.d.
##EQU00002##
Solving equations (1) and (3) simultaneously produces equations
(4a) and (4b) which show that the ratio of coefficient x.sub.r to
coefficient x.sub.d and its inverse define relationships between
focal length f and focal lengths f.sub.r and f.sub.d,
respectively.
.times..times..times..times. ##EQU00003##
For both the refractive surfaces and the DOEs, lens 10 may use
polycarbonate material, for which expansion coefficients x.sub.r
and x.sub.d have the following values:
x.sub.r=246(.times.10.sup.-60C.sup.-1) (4c)
x.sub.d=131(.times.10.sup.-60C.sup.-1) (4d)
Equations (5a) and (5b) below show that substituting the
polycarbonate coefficient values into equations (4a) and (4b)
produces a directly proportional relationship between focal length
f and focal lengths f.sub.r and f.sub.d, respectively.
.times..times..times..times. ##EQU00004##
.times..times..times..times. ##EQU00005##
Where lens 10 uses acrylic material, the following values and
equations apply. x.sub.r=315(.times.10.sup.-60C.sup.-1) (5c)
x.sub.d=129(.times.10.sup.-60C.sup.-1) (5d)
.times..times..times..times..times..times..times..times.
##EQU00006##
Thus, where the hybrid lens has positive power (i.e., has a focal
length greater than zero) and uses a material (e.g., polycarbonate
or acrylic) for which refractive surfaces are more sensitive to
temperature changes than DOEs (i.e., the value for coefficient
x.sub.r is greater than the value for coefficient x.sub.d), the
hybrid lens has the general shape of a lens with negative power.
However, in such a lens, the positive power of the DOEs overcomes
the negative power of the refractive surfaces, to produce a net
positive power For the lens. In at least some cases, such a lens
can use DOEs that are small relative to the size of the lens.
FIGS. 2A and 2B illustrate lenses 20 and 22 of polycarbonate and
acrylic, respectively, which lenses are other embodiments of the
hybrid lens and in each of which substantially all of the negative
power of the hybrid lens is provided by one of the surfaces 12' or
12'' and substantially all of the positive power is provided by
another of the surfaces 14' or 14''.
FIG. 3 shows a lens 24 that is another embodiment of the hybrid
lens and in which one of the surfaces 12''' provides not only
substantially all of the negative power but also substantially all
of the positive power, and the other surface 14''' provides no
significant negative or positive power. As shown in FIG. 3, the one
surface may include a DOE that is substantially spherical in
average and the other surface may be substantially flat and may be
used for aspherical replication.
FIG. 4 shows a lens 26 that is another embodiment of the hybrid
lens and in which one substantially spherical surface 12''''
provides less of the refractive power than another substantially
spherical surface 14'''', and substantially all of the diffractive
power is provided by a surface-relief DOE on the other
substantially spherical surface 14''''. Surface 12'''' may have an
aspherical surface or replica.
Where the two surfaces of the hybrid lens contribute substantially
equally to the diffractive power, a size increase amounting to a
factor of four may be achieved for features of the DOEs without a
significant loss in resistance to optothermal changes.
In at least some cases, because acrylic requires less refractive
and diffractive power than polycarbonate for the same focal length
f as revealed by equations (5a), (5b), (6a), (6b) above, it may be
advantageous for the hybrid lens to be constructed of acrylic
material instead of polycarbonate material.
FIG. 5B shows a lens 30 that is another embodiment of the hybrid
lens, which embodiment includes an aspherical mold that is pressed
from a drop of polymer to form an axicon 32 on a substantially
spherical surface 34 of the lens. The lens 30 also includes a DOE
36 formed in another surface 38 of the lens. FIG. 5A provides a
conceptual illustration of lens 30.
The DOE 36 may have eight phase levels 40a h as illustrated by FIG.
6 which for clarity shows DOE 36 in a flat profile, not in the
actual convex profile provided in accordance with the athermal
aspect of the hybrid lens as described above.
The axicon enhances the ability of the hybrid lens to focus laser
beams to achieve elongated profiles advantageous for bar-code
scanning, as described below.
FIG. 7 shows a lens 42 that is another embodiment of the hybrid
lens, which embodiment has an aspherical surface 34' that has the
optical properties of surface 34 combined with axicon 32. Thus lens
42 performs similarly to lens 30 but is a single piece and
therefore may be less expensive to manufacture.
Lenses 30 and 42 may be made of polycarbonate which has properties
described above.
A lens-axicon combination may be particularly useful for extending
the working range (e.g., by 50 100%) of a CCD-based bar code
scanner. In the combination, the axicon operates as a phase
correction element to allow the scanner to resolve an out-of-focus
bar code that the scanner could not resolve by relying on the lens
alone.
FIG. 8 illustrates lens 44 and axicon 46 which together are an
example combination 48 of the lens-axicon combination. Combination
48 has an aperture 50 that has a diameter 1 and is a distance a
from a CCD imager 52 of a bar code scanner, a distance b from an
in-focus point 54, and a distance z from a barcode-bearing surface
58 at a surface point 56. The lens 44 may be a doublet, a Cook
triplet anastigmat or a symmetric double Gaussian, and provides
optical power to bend incident light toward the imager 52. By
providing a longitudinal spherical aberration, the axicon 46
effectively elongates the focal depth of the lens 44 by
contributing phase correction when the surface 58 is not at the
in-focus point 54. The axicon 46 has an axicon induced phase
coefficient .alpha..
Equation (7) describes an MTF value as a function of spatial
frequency v (e.g., of a bar code symbol) for a lens having an
axicon that includes a circular pupil of diameter 1, and has polar
coordinate values .rho. and .theta. with an origin at the pupil's
center, and a normalized radial coordination value v (i.e., half of
the product of .rho. and diameter 1), where .lamda. represents the
wavelength and .lamda. represents the wave number (i.e., 2.pi.
divided by the wavelength .lamda.).
.function..nu..pi..times..intg..theta..theta..pi..times.d.theta..times..t-
imes..intg..times..times..times..times..theta..times..times..times..times.-
.theta..times..times..function..times..times..times..times..times..times..-
times..times..theta..alpha..function..times..times..times..times..times..t-
imes..times..theta..times..times..times..times..times..times..times..times-
..theta..times..times.d ##EQU00007##
FIGS. 9A and 9B show modulation transfer function ("MTF") curves
MTF1a, MTF2a, MTF3a and MTF1b, MTF2b, MTF3b, respectively, each of
which describes the sharpness of an image of a bar code symbol as a
function of the distance z, for a particular value (i.e., 0,
-0.0003, or -0.001) for the axicon induced phase coefficient
.alpha. and a particular spatial wavelength (i.e., 10 mil or 20
mil) of the bar code symbol. A high MTF value represents a
substantially in-focus image at the imager, and an MTF value near
zero represents an image that is almost completely out of focus. In
general, data can be derived from an image of a bar code symbol
more accurately if the image is sharper.
As shown in FIG. 9A, where the spatial wavelength is 10 mil and the
axicon induced phase coefficient .alpha. has a value of 0 (i.e.,
where there is effectively no axicon), curve MTF1a shows that the
MTF value peaks at about 0.75 at a z distance of about 5 inches,
and remains below 0.2 for any z distance greater than 11 inches. By
contrast, as shown by curve MTF2a, the use of an axicon having a
value of -0.003 for the axicon induced phase coefficient .alpha.
changes the optical characteristics of the lens-axicon combination
so that the MTF value peaks at about 0.5 at a z distance of about
9.5 inches, and remains above 0.2 in a z distance range from about
4 inches to about 16 inches. Thus, for example, if data can be
derived accurately from a bar code symbol image that has a
sharpness corresponding to an MTF value of 0.2 or greater, for a
bar code symbol having a spatial wavelength of 10 mil the axicon
allows data to be derived from a distance of up to about 16 inches,
which is about 5 inches further than data can be derived without
the axicon.
FIGS. 9A, 10A 10B, and 11A 11B illustrate MTF curves MTF1b MTF3f
for other values for the axicon induced phase coefficient .alpha..
FIGS. 12 and 13 show other MTF curves that describe the sharpness
of an image of a bar code symbol as a function of a normalized
spatial wavelength v for several values for the axicon induced
phase coefficient .alpha. and several values for focusing error
w.
Other embodiments are within the scope of the following claims. For
example, each lens may be formed from separate pieces (e.g.,
refractive lens and DOE pieces) or may be formed as a single unit.
Other types of plastic may be used. In each lens, refractive or
diffractive power may be distributed in any way that renders the
lens substantially athermalized.
* * * * *